When you call a function, you pass some responder to the call. If the responder doesn't exists, then you get this error.
Look in
http://www.flash-db.com/Tutorials/hello/flashamfphp.php?page=3 at the code:
[script]
//This is the object who handles request
request = new Object(); //handles result
request.onResult = function(result){
show.text = result;
}
......
service.makeEcho(request, "Hello World!");
[script]
When the service makeEcho is called, it pass the request object to handle the response. Since request object was defined and also his onResult, then it shows in the textbox. Check if your request object exists and follow the code as exposed in the tutorial
Jorge